1. Field of the Invention
The present invention relates to a hydrotreating catalyst containing molybdenum (Mo). Particularly, it relates to a hydrotreating catalyst having high desulfurization activity and desulfurization selectivity when used in hydrotreatment in which a sulfur content in catalytic cracking gasoline is reduced and also hydrogenation of olefins and aromatics is suppressed.
2. Brief Description of the Background Art
Form the viewpoint of environmental protection of city and roadside air and reduction of environmental burden on a global scale, it has been required to clean exhaust gas from automobiles and reduce the discharge of carbon dioxide. With regard to gasoline-powered automobiles, in order to reduce the discharge of carbon dioxide, it is required to improve fuel economy. Thus, thereafter, it is expected that high mileage automobiles having a direct-injection engine or a lean burn engine may increase. For these automobiles, a nitrogen oxide-reducing catalyst working under oxygen-excessive conditions (lean conditions) is necessary, which is different from a ternary catalyst which is a conventional nitrogen oxide-reducing catalyst. However, since the nitrogen oxide-reducing catalyst working under oxygen-excessive conditions (lean conditions) is poisoned by sulfur content and cleaning performance lowers, it is still required to achieve further decrease of the sulfur content in gasoline.
Currently, commercially available gasoline is prepared by mixing various base materials so as to satisfy the standard of an octane number. Among the materials, a catalytic cracking gasoline obtained by cracking, in a catalytically cracking apparatus, vacuum gas oil and atmospheric residue whose uses are limited as they are is a base material having a high octane number and a mixing ratio thereof in the commercially available gasoline is as high as 40 to 70%. However, since starting materials of the catalytic cracking gasoline are vacuum gas oil and atmospheric residue containing a large amount of sulfur, the sulfur content of the resulting catalytic cracking gasoline is also higher than other base materials for gasoline. Therefore, in order to reduce the sulfur content of the commercially available gasoline produced by mixing various base materials, it is indispensable to desulfurize the catalytic cracking gasoline which is a main base material.
For reducing the sulfur content of the catalytic cracking gasoline, it is common to use the vacuum gas oil and atmospheric residue after hydrodesulfurization as starting oils for catalytic cracking. However, the apparatus for hydrodesulfurization of these heavy oils is an apparatus requiring a high temperature and a high pressure and hence there exist many technical and economical problems in ultra-deep desulfurization of the staring oils.
Sulfur compounds contained in the catalytic cracking gasoline can be hydrodesulfurized by an apparatus requiring relatively low temperature and low pressure. Therefore, if the catalytic cracking gasoline can be directly hydrodesulfurized, there are advantages that the process may be not only economically inexpensive but also can respond the case where the sulfur content of the starting oils for catalytic cracking is high. However, when the catalytic cracking gasoline is hydrodesulfurized with a conventional desulfurization catalyst, there arises a problem that olefins contained in the catalytic cracking gasoline as components having a high octane number are also hydrogenated to result in decrease of the octane number.
In order to improve the problem, with regard to the hydrotreatment of the catalytic cracking gasoline, there are a technique of separating the starting oil into a light component and a heavy component by distillation and hydrodesulfurizing each component under different conditions (JP-A-8-209154, U.S. Pat. Nos. 4,990,242, and 5,318,690); a process for reducing the sulfur content by reacting diolefins and mercaptans contained in the catalytic cracking gasoline in minute amounts to convert them into sulfides and removing the sulfides by distillation (WO97/08272 (corresponding to JP-T-2001-519834)); a process for hydrodesulfurization after converting terminal olefins into inner olefins having a low reactivity (JP-A-9-137172); a process comprising hydrodesulfurization involving the decrease of the octane number by hydrogenation of olefins in the first step and subsequently recovering the octane number by isomerization with a solid acid catalyst in the second step (WO95/10850 (corresponding to JP-T-9-503814) and WO96/07713 (corresponding to JP-T-10-505381)); and the like. However, there are problems that the suppression of hydrogenation of the olefins is not sufficient, the decrease of the sulfur content is limited since kinds of sulfur compounds capable of being reduced are restricted, and other facilities are necessary since a multi-step and complex process is required. Therefore, it is desired to develop a hydrotreating catalyst low in hydrogenation activity against olefins in the catalytic cracking gasoline and excellent in desulfurization.
With regard to a selective desulfurization catalyst for the catalytic cracking gasoline for solving such problems, there have been proposed a method of using a catalyst modified with an alkali metal (JP-A-8-277395); a method of preventing the decrease of the octane number by the combination with a zeolite catalyst (U.S. Pat. No. 5,352,354 and WO00/29510 (corresponding to JP-T-2002-530469)); a method of using a catalyst subjected to a certain pretreatment (U.S. Pat. No. 4,149,965); a method of using a hydrodesulfurization catalyst prepared using an organic carboxylic acid (JP-A-2003-286493); a method of using a catalyst obtained by laminating a certain amount or more of an active metal as a catalyst (JP-A-2003-299959), and the like. However, since hydrogenation depth of olefins is also enhanced simultaneously when the desulfurization activity against the catalytic cracking gasoline is tried to be enhanced, there arises a problem that the decrease of the octane number cannot be sufficiently suppressed.
With regard to the hydrotreating catalyst for the catalytic cracking gasoline for solving such problems, there have been conventionally widely used sulfide catalysts obtained by preparing an impregnating solution comprising a compound of molybdenum, a compound of a Group 8 metal of the periodic table, such as cobalt or nickel, and/or a phosphorus compound and supporting the solution in an oxide form on a porous inorganic oxide support such as alumina, silica, or zeolite, followed by activation through presulfuration. The active sites for desulfurization in these sulfide catalysts are considered to be sulfur-coordinating unsaturated sites exhibited on a Group 8 metal-Group 6 metal-S structure (e.g., Co—Mo—S structure) formed at the edge site of a sulfide layer of molybdenum or tungsten which is a Group 6 metal. In order to improve the desulfurization activity, an attempt to highly disperse the Group 6 metal sulfide has been carried out. However, since there are no attempt to quantitatively determine the degree of high dispersion of the group 6 metal sulfide and to correlate the degree with desulfurization properties of the catalytic cracking gasoline and reaction properties of olefins, there exists no sufficient guidelines for designing a catalyst for hydrotreatment of the catalytic cracking gasoline and hence it is difficult to respond requests for further improvement of the desulfurization performance and suppressing hydrogenation of olefins.